Many electronic devices such as desktop computers, laptop computers, tablets and smart phones employ integrated and/or discrete semiconductor memory devices to store information. Such semiconductor memory devices fall into either volatile or non-volatile categories. Volatile memories lose stored information when power is removed, while non-volatile memories retain their stored information even when power is removed. Volatile memories include random access memory (RAM), which is further divided into sub-categories including static random access memory (SRAM) and dynamic random access memory (DRAM).
A typical DRAM memory cell has only one transistor and one capacitor, so it provides a high degree of integration for bulk information storage; however, DRAM requires constant refreshing and its slow speed tends to limit DRAM to computer main memories. On the other hand, an SRAM cell design, such as a 4 transistor design (4T) or a 6 transistor design (6T), use more transistors to make the SRAM cell bi-stable, meaning the SRAM cell maintains a binary output state indefinitely, as long as adequate power is supplied. While SRAM has a lower degree of integration than DRAM, SRAM can operate at a higher speed and with lower power dissipation than DRAM, so computer cache memories tend to use SRAMs. Other SRAM applications include embedded memories and networking equipment memories. While SRAM is often selected over DRAM when faster performance is important, even faster SRAM performance is desirable. Another reason SRAM is selected over DRAM is SRAM standby current is much lower than DRAM refresh current, giving SRAM an advantage where power consumption is an issue, such as in battery-powered mobile device applications. Thus, while even faster SRAM performance is desirable, maintaining or lowering SRAM power consumption is also desirable in many SRAM applications.